1. Field of the Invention
This invention relates to a process for removing nitrogen oxides from boiler flue gas, nitric acid plant effluent gas, steel-making plant flue gas, etc., evolving from stationary sources such as fossil fuel-fired power stations, chemical factories, etc. through reduction of the nitrogen oxides to nitrogen.
2. Description of the Prior Art
One of processes for removing nitrogen oxides, which will be hereinafter referred to as "NO.sub.x ", from flue gas is a reduction process, where the flue gas is treated with various reducing agents to convert NO.sub.x to harmless nitrogen. An advantage of the process is non-production of by-products. The reduction process is further subdivided into two groups. One groups is the so-called direct reduction method, wherein the flue gas is treated with a reducing agent at a high temperature without using any catalyst, and another is the so-called catalytic reduction method, where the reduction is carried out with a catalyst. The direct reduction method is carried out at a temperature of 600.degree. C or higher for treating NO.sub.x, and therefore heat must be supplied from outside to treat a flue gas evolving at a temperature below said treating temperature as in the case of the boiler flue gas from power stations. The catalytic reduction method can be carried out at a lower temperature than that of the direct reduction method, and therefore the catalytic reduction method is most suitable for treating the flue gas at a temperature below 350.degree. to 400.degree. C, as in the case of the flue gas from power stations and industrial boilers.
Reducing agents used in the catalytic reduction method include hydrogen, carbon monoxide, hydrocarbons such as methane, propane, etc., ammonia, etc. When the reducing agent is an ammonia gas, ammonia selectively reacts with the nitrogen oxides in the flue gas. Therefore, it seems that, when a flue gas containing oxygen such as a boiler flue gas, etc. is treated according to the catalytic reduction method, it is most suitable to use ammonia as the reducing agent. The effect of the catalytic reduction method using ammonia as the reducing agent entirely depends upon a catalyst. The catalyst usually used in the catalytic reduction method is a catalyst of the platinum group such as platinum, palladium, etc. However, it is not suitable to use a catalyst of the platinum group in reducing boiler flue gas by ammonia. That is, the boiler flue gas contains sulfur oxides such as SO.sub.2 and SO.sub.3, oxygen, carbon dioxide, steam, etc. in addition to nitrogen oxides (NO.sub.x), and the catalyst is poisoned by the sulfur oxides, and the activity of the catalyst is lowered. Furthermore, the catalyst of the platinum group is expensive, and especially when a larger amount of the catalyst is required, as in the case of the treatment of the boiler flue gas, the catalyst of the platinum group is economically disadvantageous.